1. Field of Invention
The present invention relates to a method for determining the absolute position of a rotor of a linear motor that is movable with respect to a stationarily part in a direction of movement, wherein an arrangement of drive magnets in the form of a plurality of drive magnets arranged adjacently in the direction of movement of the rotor is provided on the rotor, and a plurality of position sensors are stationary arranged at a distance from one another on the stationary part in the direction of movement of the rotor, wherein a magnetic field of a drive magnet of the arrangement of drive magnets in the area of a position sensor is detected by a position sensor. The invention further relates to a linear motor with determination of the absolute position according to the invention.
2. Discussion of Background Information
Linear motors are characterized in that a movable part (the rotor) is moved with respect to a stationary part (the stator) as the result of interacting magnetic fields. For this purpose, a drive magnet (electromagnet or permanent magnet) is situated on one of the two parts, and interacts with the magnetic field, which is generated by an energized drive coil on the other part, in order to generate a linear propulsion force. When a voltage is applied to the drive coil, a magnetic field results which interacts with the magnetic field of the drive magnet, thereby generating a force on the movable part, which moves the movable part. To move the rotor, a moving magnetic field is generated by appropriate actuation of the drive coils. This basic motor principle is of course well known, and therefore does not require further discussion here. In principle it is also irrelevant whether the drive coils are provided on the moving part (rotor) or the stationary part (stator).
In order to control the movement of the rotor of a linear motor, it is absolutely necessary to know its actual position relative to the stator to be able to correctly energize the drive coils in order to generate the moving magnetic field. Therefore, determining the position of the rotor plays an important role. It is particularly difficult to determine the actual position of the rotor when the linear motor is being switched on, since it is not possible to know the location of the rotor in advance at the time of switching on. Various methods have already been proposed for position determination when the linear motor is being switched on.
U.S. Pat. No. 7,932,684 B2 describes a linear motor, for example, which for position determination additionally includes position magnets situated on the rotor and stationary position sensors (situated on the stator, for example). When the rotor is moved, the position magnets move relative to the position sensors, and the actual position of the rotor relative to the stator may be determined. The position magnets include a first series of a number of adjacently situated permanent magnets that interact with an incremental sensor, and a second series of a number of adjacently situated permanent magnets that interact with an absolute sensor. The absolute sensor, for example a Hall sensor, is designed in such a way that it provides only two states, whereby the state is changing at a defined position of the rotor. The incremental sensor, for example a magnetoresistive sensor, is designed in such a way that it provides many recurring sensor cycles, whereat the position may be determined very accurately within a sensor cycle. During switching on, it is first necessary to carry out “homing”, i.e., referencing of a predefined, known zero position. For this purpose, the rotor is moved until the absolute sensor detects a state change, with which the zero position is determined. Starting from the zero position, the actual position of the rotor may then be incrementally determined by counting the number of sensor cycles, and carrying out a fine determination of the position within the sensor cycles. In U.S. Pat. No. 7,932,684 B2, a reference run is therefore necessary for determining the position of the rotor during switching on. However, achieving this type of position detection is meaningful only for relatively limited ranges of motion of the rotor. For many applications, in particular for linear motors having a large range of motion, or for long stator linear motors having a plurality of rotors, this type of position determination is of course totally unsuitable.
The determination of an absolute position, also when the linear motor is being switched on, is ascertainable from U.S. Pat. No. 7,994,742 B2. An elongated position magnet is situated on the rotor over the possible range of motion, and is arranged in such a way that a position-dependent offset in the transverse direction results. A position sensor which detects the magnetic field of the position magnet is situated on a stationary structure, for example the stator. Due to the offset, at each position of the rotor a distinct magnetic field results which is detected by the position sensor. A conclusion concerning the actual position of the rotor may thus be immediately drawn, without movement of the rotor, also when the linear motor is being switched on. However, the range of motion is of course restricted to the length of the position magnet, and therefore is very limited. For many applications, in particular for linear motors having a large range of motion, or for long stator linear motors having a plurality of rotors, this type of position determination is of course totally unsuitable.
U.S. Pat. No. 6,876,107 B2 describes a known long stator linear motor as a linear motor. Such a long stator linear motor includes a plurality of drive coils, stationarily situated next to one another, which form the stator of the long stator linear motor. A plurality of rotors that may be moved along the stator may be situated along the stator. Each rotor bears a drive magnet. In order to move the rotor, the drive coils which at that moment interact with a rotor are energized. In this way, individual rotors may be moved independently of one another along the stator. Such long stator linear motors are frequently used in flexible transport systems, for example in a production process or in conveying technology. U.S. Pat. No. 6,876,107 B2 also describes the determination of a true absolute position, which allows the exact position of a rotor to be determined immediately when the long stator linear motor is being switched on, without having to carry out referencing (for example, by a reference run of the rotor). This is of course very advantageous, in particular considering that it is not uncommon for several hundred rotors to be simultaneously present in a long stator linear motor. For this purpose, exactly one additional position magnet on a rotor, and a plurality of position sensors, for example magnetoresistive sensors, which detect the magnetic field of the position magnet, is/are situated along the stator. However, the position sensors must be arranged closely enough together to ensure that at any point in time, at least one position sensor can detect the magnetic field of the position magnet. When the long stator linear motor is switched on, at least one position sensor thus responds for each rotor, thereby enabling position determination without referencing the rotor. The disadvantage is that an additional position magnet is necessary, and the position sensors must be situated very closely together, which requires a large number of such position sensors.